Controlling the strain and the number and type of defects in heteroepitaxial semiconductor structures is crucial for obtaining high-quality semiconductor materials with the desired properties for forming high-performance devices. In the silicon/silicon-germanium hetero-system, appropriate control of the strain in the semiconductor layers provides control over band offsets and carrier mobility. Typically, it is desirable to be able to obtain a strain-relaxed SiGe virtual substrate on which a thin strained Si film is formed. The main obstacle to obtaining high-quality strained silicon is initially in obtaining a perfect strain-relaxed SiGe film. The typical approach for relaxing a SiGe film is to grow thick strain-graded SiGe on bulk silicon substrates. In such structures, relaxation occurs by the movement of dislocations through the SiGe film. Misfit dislocations at the interfaces between the layers relax the strain in the strained layers. The difficulty with dislocation-driven relaxation is that every misfit dislocation has two threading arms associated with the dislocation. The threading arms extend through the entire film system, including the strained-Si layer, degrading the carrier mobility. The density of dislocations in a carefully engineering graded SiGe substrate is low enough to create strained-Si devices, but it would nonetheless be preferable to be able to provide a SiGe substrate with no or very few dislocations. A further limitation with the use of graded SiGe substrates is that to achieve few dislocations in the Si layer, it is necessary to grow very thick graded SiGe films. However, the relatively thick films make it difficult to have adjacent devices that are in different strain states. Another consequence of growing a relaxed buffer layer is that the relaxation process generates steps through the opening of dislocation loops on the surface. These steps have a tendency to bunch together, creating a rough growth front. A polishing and cleaning step must be carried out before growing the silicon layer, adding cost to the processing. Thermal conductivity is also poor through the graded layers.
In one approach to overcoming the limitations of dislocation-based relaxation, compliant substrates have been proposed as a way to achieve strain control with no dislocations. A compliant substrate is composed of a very thin strained-layer system supported by a thick rigid substrate. The strained system is intended to distribute the strain based on thicknesses of the different layers by sliding on a compliant substrate. In a compliant system, all of the layers would have the same in-plane lattice constant but would be in different strain states. The problem with the compliant-substrate approach is that it is very difficult to achieve a truly compliant substrate, since doing so requires a large-scale slipping between the strained system and the compliant layer.
Another approach to obtaining the effect of a highly compliant substrate is to form patterned mesas that are undercut to leave each mesa supported by a center pedestal. The mesas can then act as free-standing substrates for SiGe growth. P. N. Mooney, et al; “Elastic Strain Relaxation in Free-Standing SiGe Structures,” Applied Physics Letters, Vol. 84, No. 7, 16 Feb. 2004, pp. 1093–1095. The supported mesas formed in this manner are fixed in location and are limited in the surface area available for device formation.